Broadband quasi-phase-matched harmonic generation in an on-chip monocrystalline lithium niobate microdisk resonator

We reveal a unique broadband natural quasi-phase-matching (QPM) mechanism underlying an observation of highly efficient second- and third-order harmonic generation at multiple wavelengths in an x-cut lithium niobate (LN) microdisk resonator. For light waves in the transverse-electric mode propagating along the circumference of the microdisk, the effective nonlinear optical coefficients naturally oscillate periodically to change both the sign and magnitude, facilitating QPM without the necessity of domain engineering in the micrometer-scale LN disk. The second-harmonic and cascaded third-harmonic waves are simultaneously generated with normalized conversion efficiencies as high as 9.9%/mW and 1.05%/mW2, respectively, thanks to the utilization of the highest nonlinear coefficient d33 of LN. The high efficiency achieved with the microdisk of a diameter of ∼30 μm is beneficial for realizing high-density integration of nonlinear photonic devices such as wavelength convertors and entangled photon sources.